The present invention relates to a novel production method of chlorinated rubber widely used for mainly heavy anticorrosion paint for outdoor buildings etc. and a novel chlorinated rubber thus obtainable.
The chlorinated rubber is a resin chlorinated natural rubber, synthetic rubber or the like up to 60% or more chlorine content. Since it dissolves into organic solvents and is excellent in the weather resistance, it is used for the heavy anticorrosion paint etc. and produced in abundance industrially. Today, for producing chlorinated rubber industrially, a method (solution method) is adopted, wherein natural rubber or synthetic rubber is dissolved into chlorine-containing solvent being inert to chlorine such as carbon tetrachloride and the chlorination is performed by introducing chlorine gas to this solution.
When performing the chlorination by solution method, not only expensive chlorine-containing organic solvent must be used in large quantities, which is uneconomic, but also complete separation and recovery of chlorine-containing solvent such as carbon tetrachloride from product are difficult, thus it admixed within product or it volatilized to escape into the atmosphere. It is pointed out that the chlorine-containing organic solvent represented by carbon tetrachloride is injurious to the human body and it has a danger to destroy the ozone layer as well, so it is very probable that it cannot be used hereafter. The chlorination by solution method adopted traditionally has therefore serious problems as described above.
Moreover, as a quite different chlorinating method from the solution method, an attempt to chlorinate the latex of natural rubber has been made. In Brit. Pat. 634241 or Industrial and Engineering Chemistry, Vol. 43, P. 2535 (1951), a method wherein cationic latex is prepared from natural rubber and chlorination is conducted by blowing chlorine gas thereinto is described. By this method, however, only a product with 61% chlorine content at maximum could be obtained and the chlorinated rubber obtained could not dissolve into solvent, thus it could not be used for paint. For obtaining chlorinated rubber soluble into solvent from this product, it was required to suspend the product with about 61% chlorine content into solvent such as carbon tetrachloride for further chlorination. Furthermore, in Japanese Patent Publication No. Sho 34-10195, a chlorinated rubber-making method wherein, after hydrochlorinated rubber is produced from natural rubber or synthetic rubber latex and then this is chlorinated up to 50 to 60%, the product is separated and chlorination is performed in the state of solid phase to dissolve into solvent is disclosed. But, the procedure is complicated and, in addition, the reaction efficiency is poor resulting in the lack of economic aspect. As described, there are many problems in the attempts to produce chlorinated rubber from latex, so that they have not been put into practice industrially.
As a result of diligent studies for solving said problems, the inventors have found that the chlorine-containing organic solvent such as carbon tetrachloride is not used at all, nevertheless the chlorinated rubber soluble into organic solvent can be produced directly from latex through following process.